You can stuff a 5-7 gallon with lots of live plants. Here's my setup as an example-
3 amazon swords
3 lace java ferns
2 java ferns
2 pieces of driftwood wrapped in riccia floutians
1 piece of driftwood wrapped in java moss
1 anubias nana
Bettas really love cover, so I tend to feel the more the better (Gyarados seems to agree with me, he spends most of his time prowling around in his greenery). Anubias, java ferns and moss and riccia are all very easy, I am new to amazon swords so I can't say too much about them yet. If you like the natural look, Petco and Petsmart sell small bundles of river rock that are very nice decorations, as well as natural gravel.
As for tankmates a 5-7 is a bit too small for more fish. Try about 4 ghost shrimp (they run 50 cents a piece). They can be sensative to ammonia and Ph, and your betta may eat them, but as cheap as they are they aren't a huge loss. If they do survive, they eat a lot of waste and are a good clean up crew for the tank. It's also fun to try and find them as they end up in the oddest places :)
Here's my 5 gallon au naturale...